1. Field of the Invention
The present invention generally relates to micro-relays. The micro-relays may be fabricated by using semiconductor fabrication techniques. The micro-relay has various marked advantages over the conventional relays, and is one of the devices that are getting most of the attention these days.
2. Description of the Related Art
Generally, the micro-relays have a movable plate that faces a stationary substrate. There is a type of micro-relay that utilizes electrostatic attraction (electrostatic force) that develops when a given voltage is applied between the stationary substrate and the movable plate. In this type of micro-relay, the movable plate may shift towards the stationary substrate due to the function of the electrostatic attraction, so that a contact can be made. The contact can be released by stopping the voltage supply. There are several proposals of the electrostatically actuated micro-relays. For example, Japanese Laid-Open Patent Application No. 5-242788 discloses a micro-relay that has a pair of stationary bodies between which a movable plate is interposed. This proposal is made by taking into consideration easy deformation of the micro-relay due to temperature change and difficulty in forming the electrodes.
However, the above proposal has a disadvantage in that hermetically sealing the interior of the micro-relay cannot be secured because wiring lines are extracted from the inside of the micro-relay to the outside. The relays are required to have a characteristic such that the ON resistance is as low as possible and is stable. Since the ON resistance is affected by the ambient atmosphere, the hermetically sealed structure is desired. The micro-relay has a great advantage in that many micro-chips are formed on a wafer using the semiconductor fabrication techniques and are divided into separate pieces by dicing, and is therefore suitable for mass production. In this case, it is required to protect the fine actuator and contacts of the micro-relay from water and scattering powders during dicing. However, in the micro-relay disclosed in Japanese Laid-Open Patent Application No. 5-242788, the hermetically sealed structure is not secured prior to dicing. Thus, there is difficulty in securing the desired relay performance.
Besides the hermetically sealed structure, the micro-relay has several problems to be solved as described below, and it is therefore desired to provide an improved micro-relay in which these problems have been solved as many as possible.
(1) The use of electrostatic force for actuating the movable contact makes it possible to realize a simple structure and low power consumption. In this type, it is important to secure the contact-to-contact distance as long as possible in order to improve isolation. As the contact-to-contact distance increases, the degree of signal leakage between the contacts decreases. It should be noted that the electrostatic force that actuates the movable plate is proportional to the square of the voltage applied and is inversely proportional to the square of the distance between the contacts. Further, the contact-to-contact distance is as very short as a few μm, when the maximum drive voltage applicable in practice (up to about 10 V) and the size of the micro-relay are considered. Consequently, it is very difficult to realize a micro-relay structure having a long contact-to-contact distance. It will be noted that the contact-to-contact distance is obtained when the movable contact is located at the home position.
(2) The micro-relay is suitable for switching of fine signals rather than conventional power switching in light of electrostatic force, contact size and contact-to-contact distance. The micro-relay is suitable for a relay of high-frequency signals (RF relay) because of easy forming of signal lines and small contacts. In the RF relay, it is particularly important to improve the isolation performance. This requires reducing the electrostatic capacitance between the contacts in the OFF state. Reducing the electrostatic capacitance may be effectively achieved by reducing the areas of the facing contacts and securing the reasonably long contact-to-contact distance.
However, reducing the areas of the facing contacts decreases the contacting area and thus increases the ON-resistance or contact resistance. Further, there is a limit on the available contact-to-contact distance. Thus, it is not easy to design the micro-relay having satisfactory RF performance.
(3) The micro-relay utilizing electrostatic attraction is equipped with driving electrodes respectively provided to the stationary substrate and the movable plate in addition to the contacts. The shorter the distance between the electrodes, the greater developing force based on the electrostatic attraction. This may cause an unwanted situation in which the movable plate is brought into contact with the stationary electrode at a position besides the original contact-made position. In this case, the movable plate is sticking to (or tightly attached to) the stationary electrode due to the residual charge between the electrodes (charge up), and is no longer detached therefrom. In this case, the micro-relay does not provide the original switching function.
(4) The contact force based on the electrostatic force is weak in the micro-relay. Generally, the relay is desired to have large contact force and small contact resistance in order to stabilize the relay. It is thus desired to provide a micro-relay that has large contact force although it is driven by a low voltage. However, such a desire for the micro-relay will not be fulfilled in practice because the large contact force and the small contact resistance are incompatible. The micro-relay is further required to accurately define the distance between the electrodes and improve the production yield. It is desired to avoid connections made outside of the micro-relay such as wire bonding and to provide an advanced structure that enables downsizing of package and reduced resistance of signal lines.